Reflecto Shield: Whenever the demon cooldown resets, add another invisible demon to orbit you with instructions to reverse vectors, force, and spin of anything that enters (or “is in” if some clever person tries to teleport something in) a sphere around your person when the object passes a certain threshold of relative speed, or at a verbal/nonverbal command. I'm imagining that multiple demons working in conjunction can impose more physical force in tandem.

Enforced Geass: Tricky if specific condition issued is prone to interpretation of demon like “If you betray me” but otherwise an intelligent summoner can bind demons onto a person’s heart or other vital organ and told to kill if condition is not upheld.

Character who is a mundie, yet is so renowned for his precise, accident free demon instructions record, that corporations and demon-summoners alike come to him to request his services in looking over demonic instruction contracts or to solve particularly intractable problems. An odd mix of ambitious lawyer and engineer. (he may or may not be bargaining for additional personal powers in clauses hidden away in his client's contracts)

Criminal Rings and Governments that test for Demonic-Summoning abilities and routinely kidnap otherwise unnoteworthy individuals to exploit their abilities

Are demons predictably malicious in the way they seek to interpret commands to do the most damage? Are they unique entities with their own preferences or are they uniform in their behavior. If you gave 5000 demons the choice of killing an old rapist or an innocent child, would every demon kill the child every time?

Perpetual Motion? Assuming demons don’t need sustenance, aren’t they capable of producing force with no energy consumption? (Or is the secret that they are drawing their power from somewhere)

Can demons kill other demons? Can people kill demons? Are they even capable of being killed or is the only way to stop them to order another demon to do the opposite of whatever the current demon is doing to negate their effect at every turn? (Though its hilarious how terribly wrong that could go)